What is the sky? How high is it? What color is it? What birds can live above it? These questions are difficult to answer, aren't they? In fact, the sky has no color. Then why does the sky look blue?

    We know that there is air around the world. When planes fly, they need air to lift their wings. Planes can't fly very high. That is because the air gets thinner when they go higher. The same will happen to us if we go high enough. If we go up about 1,200 kilometers from the earth, we'll find no air there. Some birds can fly into the sky as high as a plane, but they cannot fly or live without air.

    Looking up above us, you will see the blue sky with white clouds. It is usually light blue. The color would be bluer when you see on the top of a high mountain. And you will see fewer clouds in the sky between you and the sun. Sometimes the sky doesn't look blue. When the sun rises and falls, you will see red, orange and other colors in the sky, such are the most beautiful moments of the day.

    Who can see the colors of the sky high above the earth? The astronauts! They can see those beautiful colors of the sky from the outer space. However, before flying into the outer space, they sit in a spaceship high above the earth and there is no air at all. The sky would be that dark and its color would be black.

    Now you may understand that we see colors because the air scatters (使散射) the sunlight. Light travels through the air. Without air there would be no colors.

    What is the hardest sport in the world? Some people say it's boxing (拳击), because boxing is dangerous and boxers have to be strong and fast. Other people say the hardest sport is soccer.

    However, most people say the hardest sport is the triathlon (三项全能运动). A triathlon is a race that includes three sports. First, racers swim. After that they ride bicycles and finally they run. In the most common triathlon, racers swim 1.5kilometres, bike 40 kilometres, and run 10 kilometres. The winner is the person who finishes the race first.

    There is another kind of triathlon too. It is called the "Ironman Triathlon (铁人三项)". This triathlon is much longer. People who finish this triathlon are called "ironmen" or "ironwomen". This is because these people are so strong that their bodies are just like iron (铁).

    In the past, there were other races including three sports. However, they didn't use the word "triathlon". That word was first used to describe a race in California in 1974.

    Triathlon racers have to train very hard for their races. Many of them swim and run. A lot of people also do things like yoga (瑜伽). They have many different ways to prepare, but they all have the same goal. They want to be in perfect shape.

    To be an ironman or an ironwoman, you have to be strong and smart. You should know how to save energy during the race.

There are many colors in nature. But do you think that a color has weight? Let's do a test.

First, put two objects with the {#blank#}1{#/blank#} weight into two boxes. Then cover the boxes. Third, wrap (包裹) one box with a red piece of paper, the other with white. Now hold the boxes {#blank#}2{#/blank#} your hand one by one. Which one is {#blank#}3{#/blank#} ? It is certain that you will think the red one is heavier. Why do you think so? A scientist did many {#blank#}4{#/blank#} and found that different colors have different weights in a man's mind.

Also, scientists say that people accept the colors they like, and {#blank#}5{#/blank#} the colors they hate. So your body and mind will be {#blank#}6{#/blank#} by using the colors you like. Or you'll be nervous {#blank#}7{#/blank#} even get ill. For example, you {#blank#}8{#/blank#} blue and hate red. If you stay in a room with red windows and wallpaper for two hours, you'll feel you have been there for {#blank#}9{#/blank#} hours. But if the room is blue, you'll feel you have been there for only an hour, And, if a person walks {#blank#}10{#/blank#} of a blue room and into a red room, his temperature will rise. That means our body temperature will change with different colors.

Rock climbing（攀岩） did not become a sport until late in the 1900s. But now it has become a popular outdoor activity. While rock climbing may still be necessary for mountain climbing，most people just climb low mountains for fun. 

If you have ever done rock climbing，you will know that it is not a very easy sport. Indoor and outdoor rock climbing both need practice. What most people don't know is that rock climbing is one of the sports that depend on your body and mind（意志）. 

To climb successfully，one must use wonderful skills. One of the important rules is to always have three parts of your body on the rock face. Another important rule is that your arms only hold you close to the rock face while your legs are pushing you up，the closer you are to the rock face，the easier it is to climb. 

Rock climbing is a wonderful sport. Once you start to learn how to climb，you'll find more fun. People who like games such as chess playing or problem—solving may love rock climbing，because they are nearly the same in the way of thinking. For anyone who wants to get into beautiful shape，rock climbing is a fun and good way. Anyone who enjoys a difficult game and loves the outdoors should think of taking rock climbing as a new hobby.